Plastication control system for injection molding machines

ABSTRACT

A plastication control system for controlling the shot size and plastication time of an injection molding machine. The control system includes a computer which calculates the screw speed based on the operating conditions of the immediately preceding cycle in order to control the plastication time to a predetermined value. Additionally, the system controls the back pressure applied to the rotating screw in order to maintain the melt temperature of the plastic at the forward end of the barrel at a predetermined value.

United States Patent [1 1 Ma et al. Mar. 19, 1974 PLASTICATION CONTROLSYSTEM FOR [56] References Cited INJECTION MOLDING MACHINES UNITEDSTATES PATENTS [75] Inventors: Carlton Y. W. Ma; John W. Peter,3.693.946 9/1972 Merritt 259/l91 both of Cincinnati, Ohio [73] Assignee:Cincinnati Milacron lnc., Primary Emmi' zerHarVey Hornsby CincinnatiOhio ASSISIdnI Examiner-Alan I. Cantor [22] Filed: Nov. 17, 1972 7ABSTRACT [21] Appl. No.: 307,294 A plastication control system forcontrolling the shot Related U S A plicafion Data size and plasticationtime of an injection molding map chine. The control system includes acomputer which [62] Division of Ser. No. 83,072, Oct 22, 1970, Pat. No.calculates the screw speed based on the Operating 3 7215 conditions ofthe immediately preceding cycle in order to control the plasticationtime to a predetermined 2% 32} 4 value. Additionally, the systemcontrols the back pres- 5 l9 10 5 1 86 187 sure applied to the rotatingscrew in order to maintain the melt temperature of the plastic at theforward end of the barrel at a predetermined value.

POSITION SIGNAL ME LT TEMPERATURE IN PUT ii---- DATA CONTROLLER l szPRESSURE CONTROL SIGNAL PATENTEMAR I 9 m4 TEMPERATURE U E M DATA MOTORSPEED BACK PRESSURE CONTROLLER PUMP ON /OFF POSITION SIGNAL PRESSURECONTROL SIGNAL SCREW POSITION TIME PLASTICATION CONTROL SYSTEM FORINJECTION MOLDING MACHINES This is a division, of application Ser. No.83,072, filed Oct. 22, 1970 now US. Pat. No. 3,721,512.

BACKGROUND OF THE INVENTION This invention relates to injection moldingmachines and, more particularly, to a control system for controlling theplastication system of such a machine so that the shot size andplastication time are automatically controlled to provide theplastication time and shot size selected by by the machine operator.

In the operation of injection molding machines, plastication isaccomplished in a heated barrel within which a screw is rotatably andtranslatably positioned. The material to be plasticated, which can be,for example, polyethylene, polystyene, polypropylene, ABS, and the like,is placed in a hopper at one end of the barrel where it is fed to thescrew by means of gravity. The screw works the material, thereby heatingit by friction, to soften it so it subsequently can be forcibly injectedinto a mold while it is in a viscous but fluid state. In addition tosoftening the material, the screw also conveys it axially along thebarrel to the point where it is discharged therefrom.

Heretofore, the set-up and operation of an injection molding machine wasaccomplished almost entirely manually and required that the operator ofthe machine set the various parameters such as screw speed, screw backpressure. shot size, and the like manually in order to provide moldedparts of the desired quality within the desired cycle operating time.Because those steps had to be accomplished manually, there was aconsiderable time period involved in setting up the machine v to mold agiven part since some of the control variables are, to a degree,interdependent. so that a change in one could very well result in achange in another.

It is thus desirable to provide a control system whereby the functionspreviously performed manually are performed automatically to assure moreuniform operating cycles and thereby provide more uniform molded parts.It is an object of the present invention to provide such an automaticcontrol system.

SUMMARY OF THE INVENTION Briefly stated. in accordance with one aspectof the present invention, control system is provided for an in jectionmolding machine having a plastication system which includes a screwrotatably and translatably positioned within a barrel for injectingplasticated material into a mold. The machine includes means forrotating and translating the screw within the barrel and the controlsystem provides means for controlling both the translation and rotationof the screw so that the plastication is achieved within the desiredtime period and so that the temperature of the resulting melt ismaintained at a constant predetermined value. Means for calculating thespeed of rotation of the screw to be employed during the next succeedingoperating cycle are provided in order to correct the system and achievethe predetermined plastication time.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of aportion of an injection molding machine showing the mold cavity andplastication system together with the several elements of the controlsystem of the present invention.

FIG. 2 is a graph showing the linear position of the screw in the barrelas a function of time and also as a function of the screw rotationalspeed. FIG. 3 is a block diagram showing in further detail thecontroller forming a part of the control system shown in FIG. 1 andillustrating the several inputs and control loops therein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawingsand particularly to FIG. 1 thereof, there is shown the plasticationsystem 10 of an injection molding machine. The system includes aseparable mold comprising a first mold portion 1 l and a second moldportion 12 which when cooperatively engaged define a mold cavity 13therebetween. Second mold portion 12 includes a passageway 14 providingcommunication between mold cavity 13 and the rear face 15 of second moldportion 12. Abutting rear face 15 of the second mold portion 12 is agenerally cylindricalbarrel 16 which can have a tapered end portionterminating in an aperture 17 which is aligned with passageway 14 andconnected therewith by means of nozzle 18. If desired a shut-off valve(not shown) can be interposed between aperture 17 and passageway 14 insecond mold portion 12 in order to permit communication between theinterior of barrel l6 and mold cavity 13 only at predetermined times. Asshown in FIG. 1, barrel 16 can include a hopper 19 into which thematerial to be plasticated is loaded.

Slidably and rotatably positioned within barrel 16 is a screw 20 whichcan be of a known construction familiar to those skilled in the art. Thescrew includes one or more external flights 21 which are helicallydisposed driven by a suitable motor 26. Driving motor 26 can be,

either electrically or hydraulically driven, but a hydraulic motor ispreferred from the standpoint of smooth operation. Also forming a partof screw 20 are a series of alternating circular grooves 27 and ridges28 which are in engagement with a gear 29 which is attached to anddrives a position transducer 30. The axial position of screw 20 ismonitored by position transducer 30 which provides an electrical signalproportional thereto. Position transducer 30 can be, for example, ashaft angle encoder, a resolver, or the like.

In operation, the material to be plasticated is placed in hopper l9 andscrew 20 is caused to rotate by means of motor 26. In the course ofrotating, screw 20 performs mechanical work on ,the material to beplasticated thereby heating it and causing it to become soft andflowable. At the same time, his also transporting the material forwardlywithin barrel 16 toward outlet aperture 17. Frequently, band typeheaters 31 are provided around the exterior of barrel 16 in order tomaintain the latter at a uniform high temperature and thereby preventheat loss from the plasticated material to the surrounding atmosphere.As the plasticated material is transported forwardly it accumulates infront of screw 20 and causes it to move rearwardly with increasingaccumulation of molten material. When screw 20 has retracted a distanceX, which provides a volume of molten plasticated material sufficient tofill mold cavity 13, the rotation of screw 20 is stopped and hydraulicpressure is applied to the rear face of piston 22 attached to screw 20,thereby causing screw 20 to move forwardly toward outlet aperture 17 andconsequently forcing the plasticated material through aperture 17,nozzle 18, passageway 14, and into mold cavity 13. Subsequently, thematerial in mold cavity 13 is permitted to cool, whereupon theplastication cycle begins once again.

As is shown in FIG. 1, the control system of the pres ent inventionincludes a controller 32, which will be hereinafter described in furtherdetail, to which input data are provided. The input data can comprisethe desired melt temperature, the desired plastication time, the desiredretraction of the screw X,,, which is related to the shot size,.and aninitial estimate of the back pressure to be maintained on screw 20 inorder to insure that the material to be plasticated is worked to theextent that it reaches a temperature adequate to permit it to readilyflow and completely fill mold cavity 13.

Information supplied to controller 32 by the system includes the melttemperature. which is sensed by a melt temperature transducer 33, themotor speed, which is sensed by a tachometer 34, the back pressure incylinder 23, which is sensed by means of a pressure transducer 35, andthe linear position of screw 20, which is sensed by position transducer30.

Outputs from controller 32 include a motor speed control signal, whichcontrols whether motor 26 is on or off and also its rotational speed,which is maintained constant during a cycle, and also a signal to a pump36, which provides hydraulic pressure to cylinder 23 in order to causescrew 20 to move forwardly and thereby inject the plasticated materialinto mold cavity 13. During the time that plastication is taking place,the back pressure within cylinder 23 is controlled by means of apressure control valve 37, such as, for example, a throttle valve, theposition of which is determined by the melt temperature in the forwardpart of the barrel. A check valve 38 is provided in the hydraulic systemto prevent back flow through pump 36 and thereby insures that pressurecontrol valve 37 controls the pressure within cylinder 23.

The position of screw 20 as a function of time is shown in FIG. 2 for agiven screw speed N. As shown, X, is the desired shot size or screwretraction distance and t is the desired plastication time. Screw speedN would be the speed necessary to operate the screw across a retractiondistance X, to achieve plastication within the time t,,. However, thelikelihood of initially selecting the proper speed is small and,therefore, the present system provides for iteration from an initiallyselected speed N shown as a dashed line in FIG. 2, until the value N isachieved, at which point the plastication system operates to provide thedesires shot size X, within the desired plastication time t,. Theiteration is accomplished while maintaining X, constant so that thechanges in screw speed provide the necessary changes in plasticationtime to achieve the desired plastication time.

Controller 32 shown in FIG. 1 is shown in further detail within thedashed lines of FIG. 3. The heart of the system is a computer means 39programmed to calculate the screw rotational speed N based on anequation which is a function of plastication time, shot size, and backpressure. For any given injection molding machine having a defined screwgeometry and utilizing a given material, the equation for screw speedcan be determined empirically. The relationship between screw rotationalspeed, shot size, plastication time, and back pressure can beapproximated as follows:

Where N is screw rotational speed in rpm X, is desires shot size ininches of screw retraction 1,, is calculated trial plastication time inseconds P is back pressure on the screw in psi and A and B are constantsfor a given screw' and material For example, for a Model 375-32CINCINNATI" injection molding machine including a screw having a 2.25:1compression ratio anda 16:1 L/D positioned within a 2.75 inch diameterbarrel and processing impact polystyrene, the equation for screwrotational speed is as follows:

N 0.334 X,/r,, 695 P, 1

Computer means 39 can be an arithmetic computer of the digital type suchas, for example, a Model CIP 2000 computer manufactured by CincinnatiMilacron Inc. Similarly other computer devices can be employed as longas they are capable of performing-the necessary computation of screwspeed utilizing the several operating parameters provided by the controlsystem.

The initial rotational speed, N for the first cycle of the machine iscalculated utilizing the input quantities of desired plastication time,desired shot size, and an initial estimate of the necessary backpressure to maintain the desired melt temperature. Computer means 39calculates a rotational speed N which is maintained constant during thatoperating cycle. When screw 20 has retracted a distance equal to X,,position transducer 30 senses that position and'provides a signal tomotor controller 41 .through a counter 40 which counts the pulsesprovided by the position transducer 30. In motor controller 41, theinstantaneous position of screw 20 is compared with X and motor 26 ispermitted to operate until screw 20 has retracted a distance equal toX,. When the latter condition has been reached motor controller 41 stopsthe rotation of motor 26. Meanwhile, the time during which motor 26 hasbeen in operation has been measured by the timer 42, which provides asignal proportional to that operating time to a time comparator 43 whichcompares the actual plastication time with the desired plasticationtime, I subtracts the two to determine the error and takes half thaterror and adds it to the time for the preceding cycle to set the trialplastication time for the next succeeding cycle. The calculated trialplastication time is then combined with the new back pressure measuredduring the preceding cycle and a new screw rotational speed N iscalculated. This speed is then used for the succeeding cycle andsubsequent corrections are applied until the actual plastication time isequal to the desired plastication time. In the meantime, a melttemperature controller 44 has been setting pressure control valve 37 tomaintain the temperature sensed by temperature transducer 33 at apredetermined value. The back pressure necessary to achieve thattemperature is measured by the back pressure transducer 35, whichprovides a signal to computer 39 through melt temperature controller 44.

Thus, computer 39 continually receives information from the injectionmolding machine and adjusts operating conditions so as to maintain theshot size and plastication time constant for the steady-state condition.Additionally, the control system also maintains melt temperature at thedesired value by controlling the back pressure applied to the screw. Itcan thus be seen that any changing condition will be immediatelyreflected in corrections to the system to maintain the operation thereofat the predetermined levels. The net result is that the parts producedby an injection molding machine which includes the control system of thepresent invention are of much more uniform, high quality than if thecontrol of the machine was accomplished manually. Furthermore, the timefor setting up the machine to the predetermined operating conditions issignifi cantly reduced since the control automatically compensates fordifferences in measured values and also any interaction effects whichmay take place when one variable is changed.

While particular embodiments of the invention have been illustrated anddescribed it will be apparent to those skilled in the art that variouschanges and modifications can be made without departing from the spiritand scope of the invention and it is intended to cover in the appendedclaims all such changes and modifications that are within the scope ofthis invention.

What is claimed is:

l. A method of controlling the operation of an injection molding machinewhich includes a plastication system comprising a plastication screwrotatably and translatably positioned within a barrel, said methodcomprising:

a. rotating said screw at a constant speed until the desired volume ofplasticated material has been processed;

b. measuring the plastication time within which said screw has beenrotated to provide said desired volume of plasticated material;

0. comparing the measured plastication time with a desired plasticationtime and generating a time correction signal proportional to thedifference therebetween;

d. computing a new screw rotational speed for the next succeedingoperating cycle based on said time correction signal to provide thedesired volume of plasticated material within the desired plasticationtime.

2. The method of claim 1 including the additional step of applying aback pressure to said rotating screw to maintain the temperature of saidplasticated material at a predetermined level.

3. The method of claim 1 wherein said speed computation is based on thefollowing formula:

where N is the screw rotational speed in rpm X, is the desired shot sizein inches of screw retraction I, is the calculated trial plasticationtime in seconds P is the back pressure on the screw in psi and A and Bare constants for a given screw and material.

1. A method of controlling the operation of an injection molding machinewhich includes a plastication system comprising a plastication screwrotatably and translatably positioned within a barrel, said methodcomprising: a. rotating said screw at a constant speed until the desiredvolume of plasticated material has been processed; b. measuring theplastication time within which said screw has been rotated to providesaid desired volume of plasticated material; c. comparing the measuredplastication time with a desired plastication time and generating a timecorrection signal proportional to the difference therebetween; d.computing a new screw rotational speed for the next succeeding operatingcycle based on said time correction signal to provide the desired volumeof plasticated material within the desired plastication time.
 2. Themethod of claim 1 including the additional step of applying a backpressure to said rotating screw to maintain the temperature of saidplasticated material at a predetermined level.
 3. The method of claim 1wherein said speed computation is based on the following formula: N AXs/tp ( B + Pb ) where N is the screw rotational speed in rpm Xs is thedesired shot size in inches of screw retraction tp is the calculatedtrial plastication time in seconds Pb is the back pressure on the screwin psi and A and B are constants for a given screw and material.